1. Field of the Invention
The present invention relates to a method and apparatus to compensate for distortion products in optical transmission systems in general and in particular to a method and apparatus to compensate for distortion products in an optical signal resulting from the interaction of a laser and an optical amplifier.
2. Description of the Prior Art
Various techniques have been used to compensate for non-linear characteristics of optical sources, i.e. lasers and modulators, in an optical signal transmission system. One such technique is called predistortion linearization. Predistortion linearization comprises the use of a non-linear device which provides an output signal which is equal in amplitude but opposite in phase with the distortion products produced in the optical sources. For example, the use of predistortion linearization of a directly modulated distributed feedback (DFB) laser and a Mach-Zehnder external modulator in an AM video transmission system was reported in a paper presented by Richard B. Childs and Vincent A. O'Byrne in the OPTICAL FIBER COMMUNICATION CONFERENCE, 1990 Technical Digest Series, Vol. 1, Conference Edition, January 1990. For the DFB laser the required predistortion circuit produced only composite second-order distortion (CSO). This was implemented using the square law transfer function of a field effect transistor (FET). The Mach-Zehnder modulator required a predistortion circuit which generated composite triple beat (CTB), i.e. third-order, distortion products with negligible second-order distortion. This was implemented using a balanced arrangement of Schottky diodes with exponential transfer functions.
The above described predistortion linearization circuits and techniques reported by Childs et al were used to compensate for distortion products in DFB lasers and modulators in an AM optical transmission system. However, so far as is known, such techniques have not been used to compensate for gain distortion products in an optical signal in the output of an optical amplifier which are produced as a result of the interaction of a laser and the optical amplifier coupled thereto. Also, the prior known compensation circuits have been generally limited to providing expansion-type gain compensation.
Expansion-type gain compensation is a compensation which compensates for a reduction in gain of the optical amplifier as a function of wavelength by increasing, i.e. expanding, the amplitude of the output signal of the laser. It has been found, however, that distortion products resulting from the interaction of a laser and an optical amplifier coupled thereto may comprise, depending on the wavelength of the laser, either expansion-type or compression-type gain distortion and thereby require either compression-type or expansion-type gain compensation, respectively. Heretofore, there has been no apparatus or method proposed which provides both types of compensation automatically and/or inexpensively.